1. Field of the Invention
The present invention relates generally to interleaver circuits and methods of interleaving data. More particularly, the present invention is directed to an interleaver circuit as required for the North American Code Division Multiple Access (CDMA) terminal according to the IS95 standard.
2. Related Art
The IS95 standard is a mobile telecommunication standard which employs Code Division Multiple Access (CDMA). CDMA is a form of modulation in which digital information is encoded in an expanded bandwidth format. In CDMA, several transmissions occur simultaneously within the same bandwidth. Mutual interference can be reduced by the degree of orthogonality of the unique codes used in each transmission. CDMA permits a high degree of energy dispersion in the emitted bandwidth.
In a mobile telecommunication environment, signal strength varies with location and movement of the mobile transmitter/receiver. Signal strength can significantly affect error rates which in turn effect the quality of communication. Due to varying signal strength, mobile telecommunication systems are susceptible to burst errors. Burst errors are groupings of errors that occur in adjacent bits of a data block as compared to errors that are dispersed over a whole block of data. The IS95 standard addresses the problem of varying signal strength and burst errors by utilizing an error correction scheme based on encoding and interleaving.
Generally, interleaving is used in conjunction with encoding (e.g., error-correcting codes) in order to lower the error rates of communication channels that are susceptible to burst errors. Interleaving is a technique in which encoded digital data is reordered before transmission in such a manner that any two successive digital data bits in the original data stream are separated by a predetermined distance in the transmitted data stream. Deinterleaving is the reverse of interleaving where data bits are reordered back to their original positions. By re-ordering the data, interleaving disperses, or randomizes, burst errors throughout the data block which improves the error-correcting capability of an encoding scheme.
According to the IS95 standard, data is encoded and interleaved prior to transmission and decoded and deinterleaved after reception. A convolutional encoder in the transmitter encodes a data block. An interleaver in the transmitter then interleaves the encoded data according to the IS95 interleaving sequence. The interleaved data is transmitted to a receiver. The receiver deinterleaves and decodes the received data. Such deinterleaving disperses burst errors, which can occur during transmission, throughout the data block. This dispersal of bit errors maximizes the capability of the decoder to correct the errors. Thus, the interleaving process is an essential part of the error protection scheme adopted by the IS95 standard.
In a mobile telecommunication system it is important to minimize chip size and current consumption. This is particularly true in the mobile stations, such as cellular telephones. Chip cost is proportional to chip size. Additionally, the more current a mobile device uses, the shorter the battery life. Thus, it is a goal in mobile telecommunication systems to reduce both chip area and current consumption.
The IS95 standard defines a formula for computing interleaving addresses. Computation of interleaving addresses using the formula defined in the IS95 standard, at the required bit rate of 19 kbs (kilobits per second), requires several MIPS (million instructions per second) of processing capability. The circuitry necessary to provide several MIPS of processing requires a significant amount of chip area and consumes a significant amount of current.
Another conventional method for computing interleaving addresses uses a look-up table to determine the proper interleaving address. This method minimizes the processing capability required to generate interleaving addresses. However, the look-up table requires a significant amount of memory. This memory space requires a significant amount of chip area to implement.
Thus, conventional implementations of mobile telecommunication systems are flawed because they utilize interleaver circuits which require a significant amount of chip area and draw a significant amount of current.